The human cranial vault contains the brain, blood vessels, and cerebrospinal fluid (CSF). The sutures of the cranium fuse by a year of age and the skull becomes a rigid structure. The architecture and physiology of the intracranial space allow for some compensation for additional intracranial volume such as hemorrhage, tumor, or excess CSF. When this compensatory capacity is exhausted, the contents act essentially as ideal fluids in a rigid container, making them subject to rapid rises in pressure when a relatively small volume of fluid is added. With sufficient rise in intracranial pressure (ICP), brain tissue is compressed and its blood supply is compromised resulting in brain damage and, if unchecked, death.
In the normal brain, CSF is secreted by tissue known as choroid plexus within cavities in the brain called ventricles. The CSF flows from the uppermost lateral ventricles through conduits into the more central third and then fourth ventricles, then flowing out of the brain to surround the spinal cord and brain. Ultimately, the CSF is absorbed on the outer surface of the brain by cells comprising the arachnoid villi. This is a continuous circulation, amounting to approximately 400 cc/day.
Any interruption in CSF circulation can result in excess CSF within the intracranial space, a condition known as hydrocephalus. In mild cases, CSF fills the ventricles excessively and stretches the cells of the brain resulting in neurological dysfunction. In severe cases, the rise in ICP is sufficient to result in brain damage and death.
The two general categories of hydrocephalus are communicating and non-communicating. Communicating hydrocephalus is caused by inability of the arachnoid villi to adequately absorb CSF. This can result from scarring due to previous hemorrhage or infection. A less-well understood form of this, known as normal pressure hydrocephalus (NPH), occurs in the elderly and is thought to be a derangement in the normal balance of CSF secretion and re-absorption. Non-communicating hydrocephalus is a consequence of mechanical obstruction of the normal flow of CSF, commonly by tumors or congenital or acquired narrowing of CSF conduits.
The most common contemporary treatment of hydrocephalus is to divert the flow of CSF. One strategy in obstructive hydrocephalus is to surgically pierce a hole in the bottom of the third ventricle, a third ventriculostomy, bypassing the obstruction. More commonly, CSF is diverted to a space in the body that has a large capacity to absorb it such as the peritoneum, pleura, or bloodstream. This strategy can be used with obstructive or communicating hydrocephalus and is accomplished by a device known as a shunt.
A shunt for CSF diversion typically consists of a synthetic tube placed through a hole drilled in the skull and passed through the brain into the ventricle. This is connected to a tube passed under the skin that terminates in the desired location. The shunt may be fitted with a valve designed to control pressure and flow as well as a device designed to mitigate over-drainage due to siphoning with upright posture.
Currently available shunt technology has several shortcomings. Valve technology is often inadequate to provide the optimal level of drainage. Under-drainage results in elevated ICP and over-drainage can result in headaches or hemorrhage due to collapse of the brain and tearing of surface blood vessels. Differential pressure based shunts, even with “anti-siphon countermeasures”, often do not adapt well to changes in posture, to fluctuating CSF production and ICP, or to changes in intracranial CSF dynamics over time. Patients with shunts and persistent headaches frequently present a challenge because it is unclear whether there is subtle over- or under-drainage. The simple externally adjustable valves available currently force the clinician to guess at the appropriate pressure setting and accept that the system cannot adapt to fluctuations in demand.
Partial or complete obstruction of shunts is common and can be due to blockage by aggregated protein, blood, or tissue invasion into the conduit as well as mechanical disconnection of the shunt system. Shunt failure is often difficult to identify until intracranial compensatory mechanisms are depleted and there is a precipitous rise in ICP constituting a surgical emergency. Detection of early shunt malfunction usually requires clinical suspicion followed by invasive testing consisting of accessing the system through the skin with a needle, measuring the pressure with a manometer, and sometimes instilling radioactive or iodinated contrast medium for radiographic imaging. These procedures can be difficult to interpret and introduce the risk of infecting or damaging the shunt, both of which can carry significant morbidity and mortality for the patient.